Field of the Invention
The present invention relates to processes for the production of hydrogen from an integrated water electrolysis cell and hydrocarbon gasification reactor.
Description of Related Art
Gasification is well known in the art and it is practiced worldwide with application to solid and heavy liquid fossil fuels, including refinery bottoms. The gasification process uses partial oxidation to convert carbonaceous materials, such as coal, petroleum, biofuel, biomass and other hydrocarbon-containing materials with oxygen at high temperature, i.e., greater than 800° C., into a synthesis gas (“syngas”), steam and electricity. The syngas consists of carbon monoxide and hydrogen and can be burned directly in internal combustion engines, or used in the manufacture of various chemicals, such as methanol via known synthesis processes, and to make synthetic fuels via the Fischer-Tropsch process.
For refining applications, the main process block is known as the Integrated Gasification Combined Cycle (IGCC), which converts the feedstock into hydrogen, power and steam. FIG. 1 shows the process flow diagram of a conventional IGCC system 100 of the prior art, which includes a feed preparation section 102, a gasification reactor 104, an air separation unit 180, a syngas quench and cooling unit 110, a water-gas shift reactor 120, an acid gas removal (AGR) and sulfur recovery unit (SRU) 130, a gas turbine 140, a heat recovery steam generator (HRSG) 150, and a steam turbine 160.
In a conventional IGCC, a feedstock is introduced via a feed line 101 to the feed preparation section 102. The prepared feedstock is then passed to the gasification reactor 104 with a predetermined amount of oxygen 103 produced from the air separation unit 180. The feedstock is partially oxidized in the gasification reactor 104 to produce a hot syngas 106 which is conveyed to the syngas quench and cooling unit 110. Hot syngas 106 is cooled with boiler feed water 156 to produce cooled syngas 114 and steam. A portion of the steam (steam 112) is used in the water-gas shift reactor 120 to produce shifted gas 122, and another portion of the steam (steam 116) is consumed in the heat recovery steam generator 150. Shifted gas 122 is treated in the AGR/SRU 130 to separate carbon dioxide 136, sulfur 138, and hydrogen syngas recovery 132. A portion of the hydrogen syngas recovery 132 (gas turbine feed 134) is passed to the gas turbine 140 with air feed 142 to produce electricity 144. High pressure discharge 146 from the gas turbine 140 is conveyed to the HRSG 150 to generate steam which is used in the steam turbine 160 to produce additional electricity 162.
The air separation unit 180 and most of the downstream processes utilize mature technologies with high on-stream reliability factors. However, the gasification reactor 104 has a relatively limited lifetime that can be as short as from 3 to 18 months, depending upon the characteristics of the feed and the design of the unit.
Three principal types of gasification reactor technologies are moving bed, fluidized bed and entrained-flow systems. Each of the three types can be used with solid fuels, but only the entrained-flow reactor has been demonstrated to efficiently process liquid fuels. In an entrained-flow reactor, the fuel, oxygen and steam are injected at the top of the gasifier through a co-annular burner. The gasification reaction usually takes place in a refractory-lined vessel which operates at a pressure of about 40 bars to 60 bars and a temperature in the range of from 1300° C. to 1700° C.
There are two types of gasifier wall construction: refractory and membrane. The gasifier conventionally uses refractory liners to protect the reactor vessel from corrosive slag, thermal cycling, and elevated temperatures that range from 1400° C. to 1700° C. The refractory wall is subjected to the penetration of corrosive components from the generation of the syngas and slag, and thus subsequent reactions in which the reactants undergo significant volume changes that result in strength degradation of the refractory materials. The replacement of refractory linings can cost several millions of dollars a year and several weeks of downtime for a given reactor. Up until now, the solution has been the installation of a second or parallel gasifier to provide the necessary continuous operating capability, but the undesirable consequence of this duplication is a significant increase in the capital costs associated with the unit operation.
On the other hand, membrane wall gasifier technology uses a cooling screen protected by a layer of refractory material to provide a surface on which the molten slag solidifies and flows downwardly to the quench zone at the bottom of the reactor. The advantages of the membrane wall reactor include reduced reactor dimensions as compared to other systems; an improved average on-stream time of 90%, as compared to an on-stream time of 50% for a refractory wall reactor; elimination of the need to have a parallel reactor to maintain continuous operation as in the case of refractory wall reactors; and the build-up of a layer of solid and liquid slag that provides self-protection to the water-cooled wall sections.
In a membrane wall gasifier, the build-up of a layer of solidified mineral ash slag on the wall acts as an additional protective surface and insulator to minimize or reduce refractory degradation and heat losses through the wall. Thus the water-cooled reactor design avoids what is termed “hot wall” gasifier operation, which requires the construction of thick multiple-layers of expensive refractories which will remain subject to degradation. In the membrane wall reactor, the slag layer is renewed continuously with the deposit of solids on the relatively cool surface. The solids that form the slag must be introduced in the hydrocarbon feed in whole or in part. Where the hydrocarbon feed contains insufficient or no ash forming material, it must be supplemented or provided entirely by a source of ash in a separate feed. Further advantages include short start-up/shut down times; lower maintenance costs than the refractory type reactor; and the capability of gasifying feedstocks with high ash content, thereby providing greater flexibility in treating a wider range of coals, petcoke, coal/petcoke blends, biomass co-feed, and liquid feedstocks.
There are two principal types of membrane wall reactor designs that are adapted to process solid feedstocks. One such reactor uses vertical tubes in an up-flow process equipped with several burners for solid fuels, e.g., petcoke. A second solid feedstock reactor uses spiral tubes and down-flow processing for all fuels. For solid fuels, a single burner having a thermal output of about 500 MWt has been developed for commercial use. In both of these reactors, the flow of pressurized cooling water in the tubes is controlled to cool the refractory and ensure the downward flow of the molten slag. Both systems have demonstrated high utility with solid fuels, but not with liquid fuels.
For the production of liquid fuels and petrochemicals, a key parameter is the mole ratio of hydrogen-to-carbon monoxide in the dry syngas. This ratio is usually between 0.85:1 and 1.2:1, depending upon the feedstock characteristics. Thus, additional treatment of the syngas is needed to increase this ratio up to 2:1 for Fischer-Tropsch applications through the water-gas shift reaction represented by CO+H2O→CO2+H2. In some cases, part of the syngas is burned together with some off gases in a combined cycle to produce electricity and steam. The overall efficiency of this process is between 44% and 48%.
Electrolysis of water is the decomposition of water into oxygen and hydrogen gas by passing an electric current through the water. FIG. 2 shows a schematic diagram of an electrochemical cell which has two electrodes: a cathode and an anode. The electrodes are placed in the water and externally connected with a power supply. At a certain critical voltage, hydrogen is produced at the cathode and oxygen is produced at the anode. The reactions proceed as follows:2H+(aq)+2e−→H2(g) (cathode)  (1)4OH−(aq)→2H2O+O2(g)+4e−(anode)  (2)
The minimum necessary cell voltage for the start-up of electrolysis, Eocell, is given under standard conditions, as represented by the following:
                              E          Cell          0                =                                            -              Δ                        ⁢                                                  ⁢                          G              0                                nF                                    (        3        )            
where ΔGo is the change in the Gibbs free energy under standard conditions;                n is the number of electrons transferred; and        F is Faraday's constant which is 96,485 C.        
There are three types of water electrolyzers categorized by the types of electrolyte used: alkaline, proton exchange membrane and solid oxide. The most common type is the alkaline electrolyzer which can be either unipolar or bipolar. FIG. 3 is a schematic diagram of a unipolar electrolyzer of the prior art. The unipolar electrolyzer resembles a tank and has electrodes connected in parallel. A membrane or a diaphragm is placed between the cathode and anode, which separates the hydrogen and oxygen as the gases are produced, but allows the transfer of ions. The bipolar electrolyzer resembles a filter press. Bipolar electrolyzers consist of many cells, as many as 100-160, which are connected in series to the circuit. Each electrode, with the exception of the two terminal electrodes, functions as a cathode at one end and an anode at the other. Electrolysis cells are connected in series, and hydrogen is produced on one side of the cell and oxygen on the other side of the cell.
In general, the operating conditions for electrolysis plants with normal or slightly elevated pressure includes an electrolyte temperature in the range of from 70 to 90° C., a cell voltage in the range of from 1.85 to 2.05 V, a power consumption in the range of from 4 to 5 KWh/m3, and a hydrogen production at a purity of 99.8% or higher. Pressure electrolysis units operate at a pressure in the range of from 6 to 200 bars, and the operating pressure has no significant influence on the power consumption. Because of its high energy consumption and substantial capital investment, water electrolysis is currently used for only 4% of world hydrogen production.
It is also known that operation of the electrolysis cell at high temperatures improves the efficiency. This optional heating step can be used to advantage when a high temperature heat source is available to increase the cell operating temperature.
While both the gasification and water electrolysis processes are well developed and suitable for their intended purposes, their combination and applications in conjunction with upgrading crude oil and its fractions do not appear to have been developed.
The problem addressed by the present invention is therefore to provide a method for the economical production of hydrogen from an integrated water electrolysis cell and hydrocarbon gasification reactor.